1. Field of the Invention
The present invention relates to noise detection and treating mechanisms and more particularly, to mechanisms designed to detect and treat bursts of bad data in a digital data stream.
2. Description of Related Art
In any data transmission event, a received signal is modified by various distortions imposed by the transmission system, together with additional unwanted signals that are inserted somewhere between transmission and reception. The latter, unwanted signals are referred to as noise. Noise is a major limiting factor in communications system performance.
There are four major categories of noise: thermal noise, intermodulation noise, crosstalk, and impulse noise.
Thermal noise is caused by thermal agitation of electrons in a conductor. It is present in all electronic devices and transmission media and is a function of temperature. Because thermal noise is uniformly distributed across the frequency spectrum it is frequently referred to as "white noise". Thermal noise cannot be eliminated; therefore, it places an upper bound on communications system performance.
Intermodulation noise is noise that is produced by some nonlinearity in a transmitter, a receiver, or in a transmission system between the transmitter and the receiver. Nonlinearity can be caused by component malfunction, excessive signal strength, or any number of other system abnormalities. Intermodulation noise produces signals at a frequency which is the sum or difference of two original frequencies or multiples of those frequencies. These derived signals can interfere with the intended signals.
Crosstalk is an unwanted coupling between signal paths. Crosstalk can occur by electrical coupling between nearby twisted pair, by coax cable lines carrying multiple signals, or by unwanted signals being picked up by antennas. Crosstalk is usually of the same order of magnitude, or less, of thermal noise.
Thermal noise, intermodulation noise, and crosstalk are all reasonably predictable and have reasonably constant magnitudes. Therefore, it is relatively easy to build communications system that can cope with them. This is not true with regard to the fourth major category of noise, impulse noise.
Impulse noise is noncontinuous, generally consisting of irregular pulses or noise spikes of short duration and relatively high amplitude. Impulse noise can be caused by external electromagnetic disturbances, such as lightning, as well as by faults or defects in the communications system. Because impulse noise is not generally predictable, and because it does not have a generally constant magnitude, it is relatively difficult to cope with.
Impulse noise is generally only a minor annoyance in analog communications system. It can corrupt voice transmission by causing clicks and crackles; however, intelligibility is rarely lost. In digital communications systems, on the other hand, impulse noise is a major problem, being a primary, if not the primary, source of error. For example, a sharp spike of 0.02 second duration can wash out one hundred bits of data being transmitted at 4800 bits per second. Thus, bursty impulse noise can cause substantial errors in digital transmission systems.
Another concept besides noise that plays a role as background to the present invention is jitter. Jitter may be time, amplitude, frequency, or phase related. Along that line, jitter is abrupt, spurious variations in the duration, magnitude, frequency, or phase of the frequency modulation of an interval, successive cycles, or successive pulses of a repetitive wave. Stated more simply, variations in pulse positions cause jitter. Jitter results from a number of causes, some of which are dependent on the pulse pattern being transmitted while others are not. Random forms of jitter are caused by noise, interference, and mistuning of clock circuits. Pattern-dependent jitter results also from clock mistuning, from amplitude-to-phase conversion in clock circuits, and intersymbol interference (ISI), which alters the position of the peaks of the input signal according to the pattern.
It is known that jitter accumulation over a digital link may be reduced by buffering the link with an elastic store and clocking out the digit stream under the control of a highly stable phase-lock loop. The elastic store is effectively a buffer that is initialized to hold a certain number of bits, and which expands and contracts to keep the bit length constant. The phase-lock loop is a device that uses feedback to minimize the deviation from one bit time to the next. It generally accomplishes this by synchronizing a variable local oscillator with the phase of a transmitted signal.
Based upon the foregoing, it should be perceived that noise and jitter are large problems in digital communications systems. Both can cause large losses of data. Although a number of steps have been taken heretofore to deal with those problems, there has yet to have been developed a system or subsystem for a communications system that is extremely effective in coping with them. Accordingly, it should be perceived that it is a shortcoming and deficiency of the prior art that such a system or subsystem has not yet been developed.